Upgrade maven to multi-project setup

We intend to add modules to file-repository that are optional.

Introduce the parent POM, multi-project setup for this project:

• file-repository-parent will contain configuration for all modules, that's the primary pom.xml
• file-repository will contain the current project code
• file-repository-ftpserver is the new module that adds FTP server support using Apache Mina https://mina.apache.org/ftpserver-project/embedding_ftpserver.html